Tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane (also referred to hereinafter as “AO-30”) is a compound widely used as an antioxidant for synthetic high-polymer materials such as polyolefins, ABS resin, and styrene-butadiene copolymers. Patent Document 1 proposes its usefulness as a storability improver in thermal recording paper. AO-30 with such great values is known to be produced, for example, through the reaction of 2-t-butyl-5-methylphenol and crotonaldehyde, as disclosed in Patent Document 2 listed below. Further, Patent Documents 3 to 5 describe that various types of crystals of AO-30, such as crystals having different crystal forms, crystals having a reduced organic solvent content, or hydrated crystals, can be produced by varying the recrystallization solvents and/or additives used during the production steps. Patent Documents 3 to 5 suggest the possibility of tackling problems arising during production, such as odor, flowability, and workability, as well as other problems such as foaming during mixing and coloring. These Patent Documents, however, describe nothing about using AO-30 for thermal recording materials.
It has long been considered that, in cases of using AO-30 as a storability improver for thermal recording paper, the use of materials having high melting points is effective in suppressing coloring of non-printing sections in the recording paper. For example, Patent Document 6 discloses a novel crystal having a high melting point which is described as improving the heat resistance of non-printing sections while maintaining the moisture-and-heat resistance of printing sections. Patent Document 6, however, does not completely solve the problem of background fogging. Accordingly, there still is a demand for further improvement in coloring suppressibility.    Patent Document 1: JP-A-58-57990    Patent Document 2: JP-B-39-4469    Patent Document 3: JP-A-56-40629    Patent Document 4: Specification of U.S. Pat. No. 4,467,119    Patent Document 5: JP-A-1-301634    Patent Document 6: Japanese Patent No. 3,816,132